mac80211: allow transmitting deauth with tainted key

When we had a connection for WoWLAN and after resume it
needed to be disconnected, the previous commit enabled
sending a deauth frame to the AP. This frame would not
go through on MFP-enabled networks as the key for it is
marked tainted before the frame is transmitted.

Allow a tainted key to be used for deauth frames. Worst
case, we'll use a wrong key because the PTK was rekeyed
while suspended, but more likely the PTK is still fine
and the taint flag really only applies to the GTK(s).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>